SOLVENT-INDUCED ELECTRON-TRANSFER AND DELOCALIZATION IN MIXED-VALENCECOMPLEXES - ELECTROCHEMISTRY

For the Ru-III/II couple in [(bpy)(2)ClOs(4,4'-bpy)Ru (NH3)(5)](3+) (b py = 2,2'-bipyridine; 4,4'-bpy = 4,4'bipyridine), E(1/2) varies linear ly with the donor number (DN) of the solvent with a slope of -26 +/- 3 mV/DN unit ranging from nitromethane to dimethylsulfoxide. For the Os -III/II couple, the variation is -3 +/- 1 mV/DN unit. Plots of Delta E (1/2) = E(1/2)(2) - E(1/2)(1) VS DN (E(1/2) is the half wave potential for the first or second wave by cyclic voltammetry) undergo a change in slope at DN similar to 14 where there is a change in oxidation stat es in the mixed-valence form from Os-III-Ru-II to Os-II-Ru-III By extr apolation of these data, Delta G degrees for the mixed-valence equilib rium, [(bpy)(2)ClOsIII-(4,4'-bpy)Ru-II(NH3)(5)](4+) reversible arrow [ (bpy)(2)ClOSII(4,4'-bpy)RU(III)(NH3)(5)](4+), varies from +5.8 kcal/mo l in nitromethane to -7.5 kcal/mol in dimethyl sulfoxide. It differs f rom Delta E(1/2) by up to similar to 20% even though it has sometimes been assumed in the literature that Delta E(1/2) = -Delta G degrees. F or [(bpy)(2)ClOs(pz)Ru(NH3)(5)](3+) (pz = pyrazine) both Os-III/II and RU(III/II) couples are significantly solvent dependent for solvents o f DN < 24. In these solvents oxidation states in the mixed-valence for m are Os-III-Ru-II. The slopes of E(1/2) VS DN plots are -21 +/- 4 mV/ DN unit (RU(III/II)) and -8 +/- 4 mV/DN unit (Os-III/II). At DN > 24 t he oxidation states switch to Os-II-RU(III) and the solvent dependence reverts to being largely in Ru-III/II There is evidence in the electr ochemical data, in comparisons between [(bpy)(2)ClOs(pz)Ru(NH3)(5)](3) and [(bpy)(2)ClOs(4,4'-bpy)RU(NH3)(5)](3+) for significant through-b ridge electronic coupling in [(bpy)(2)ClOsII(pz)Ru-III(NH3)(5)](4+) bu t not in I(bpy)2ClOs''(pz)Ru'''(NH3)51''+. The difference in behavior is caused by extensive H-bonding to the solvent at -RU(III)(NH3)(5)](3 +) in [(bpy)(2)ClOsII(pz)Ru-III(NH3)(5)](4+) This mixes solvent charac ter into d pi(Ru-III) which decreases electronic coupling across the b ridge.